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<p>从明确的构造函数实参的集合初始化对象。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <span class="t-spar">对象</span> <code><b>(</b></code> <span class="t-spar">实参</span> <code><b>);</b></code>
<p><span class="t-spar">T</span> <span class="t-spar">对象</span> <code><b>(</b></code> <span class="t-spar">实参1, 实参2, ...</span> <code><b>);</b></code>
</p>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <span class="t-spar">对象</span> <code><b>{</b></code> <span class="t-spar">实参</span> <code><b>};</b></code>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <code><b>(</b></code> <span class="t-spar">其他对象</span> <code><b>)</b></code>
<p><span class="t-spar">T</span> <code><b>(</b></code> <span class="t-spar">实参1, 实参2, ...</span> <code><b>)</b></code>
</p>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>static_cast&lt;</b></code> <span class="t-spar">T</span> <code><b>&gt;(</b></code> <span class="t-spar">其他对象</span> <code><b>)</b></code>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>new</b></code> <span class="t-spar">T</span><code><b>(</b></code><span class="t-spar">实参列表, ...</span><code><b>)</b></code>
</td>
<td> (5)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">类</span><code><b>::</b></code><span class="t-spar">类</span><code><b>()</b></code> <code><b>:</b></code> <span class="t-spar">成员</span><code><b>(</b></code><span class="t-spar">实参列表, ...</span><code><b>)</b></code> <code><b>{</b></code> <span class="t-spar">...</span> <code><b>}</b></code>
</td>
<td> (6)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>[</b></code><span class="t-spar">实参</span><code><b>](){</b></code> <span class="t-spar">...</span> <code><b>}</b></code>
</td>
<td> (7)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>在下列场合进行直接初始化：
</p>
<div class="t-li1"><span class="t-li">1)</span> 以表达式<span class="t-rev-inl t-since-cxx11"><span>或花括号初始化器列表</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>的非空带括号列表初始化</div>
<div class="t-li1"><span class="t-li">2)</span> 以花括号环绕的单个初始化器初始化一个非类类型对象（注意：对于类类型和其他使用花括号初始化器列表的初始化，见<a href="list_initialization.html" title="cpp/language/list initialization">列表初始化</a>）</div>
<div class="t-li1"><span class="t-li">3)</span> 用<a href="explicit_cast.html" title="cpp/language/explicit cast">函数式转型</a>或以带括号的表达式列表初始化<span class="t-rev-inl t-until-cxx17"><span>纯右值临时量</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>纯右值的结果对象</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span></div>
<div class="t-li1"><span class="t-li">4)</span> 用 <a href="static_cast.html" title="cpp/language/static cast">static_cast</a> 表达式初始化<span class="t-rev-inl t-until-cxx17"><span>纯右值临时量</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>纯右值的结果对象</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span></div>
<div class="t-li1"><span class="t-li">5)</span> 用带有非空初始化器的 new 表达式初始化具有动态存储期的对象</div>
<div class="t-li1"><span class="t-li">6)</span> 用构造函数<a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">初始化器列表</a>初始化基类或非静态成员</div>
<div class="t-li1"><span class="t-li">7)</span> 在 lambda 表达式中从按复制捕获的变量初始化闭包对象的成员</div>
<p>直接初始化的效果是：
</p>
<ul><li> 如果 <code>T</code> 是数组类型，那么
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx20"><td>
<dl><dd><ul><li> 程序非良构
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td></tr>
<tr class="t-rev t-since-cxx20"><td>
<dl><dd><ul><li> 按<a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">聚合初始化</a>初始化数组，但允许窄化转换，并<a href="value_initialization.html" title="cpp/language/value initialization">值初始化</a>任何不带初始化器的元素。
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw1">explicit</span> A<span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
A a<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="br0">(</span>A<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：以 A(1) 初始化 a[0] 并以 A() 初始化 a[1]</span>
A b<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="br0">{</span>A<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 错误：从 {} 隐式复制初始化 b[1] 选择了 explicit 构造函数</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<ul><li> 如果 <code>T</code> 是类类型， 
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<dl><dd><ul><li> 如果初始化器是<a href="value_category.html" title="cpp/language/value category">纯右值</a>表达式且类型与 <code>T</code> 为相同的类（忽略 cv 限定），则用初始化器表达式自身，而非从它实质化的临时量，初始化目标对象：<br>（ C++17 前，编译器可以在此情况下消除源自纯右值的构造，但适合的构造函数仍必须可访问：参阅<a href="copy_elision.html" title="cpp/language/copy elision">复制消除</a>）
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<dl><dd><ul><li> 检验 <code>T</code> 的构造函数并由重载决议选取最佳匹配。然后调用该构造函数初始化对象。
</li></ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<dl><dd><ul><li> 否则，如果目标类型是（可能有 cv 限定）的聚合类，则按<a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">聚合初始化</a>中所述进行初始化，但允许窄化转换，不允许指派初始化器，不延长引用所绑定到的临时量的生存期，不进行花括号消除，并<a href="value_initialization.html" title="cpp/language/value initialization">值初始化</a>任何无初始化器的元素。
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span>
  <span class="kw4">int</span> a<span class="sy4">;</span>
  <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> r<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">10</span><span class="sy4">;</span>
 
B b1<span class="br0">{</span><span class="nu0">1</span>, f<span class="br0">(</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>               <span class="co1">// OK：延长生存期</span>
B b2<span class="br0">(</span><span class="nu0">1</span>, f<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// 良构，但有悬垂引用</span>
B b3<span class="br0">{</span><span class="nu16">1.0</span>, <span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span>               <span class="co1">// 错误：窄化转换</span>
B b4<span class="br0">(</span><span class="nu16">1.0</span>, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// 良构，但有悬垂引用</span>
B b5<span class="br0">(</span><span class="nu16">1.0</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>n<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// OK</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<ul><li> 否则，如果 <code>T</code> 是非类类型但源类型是类类型，则检验源类型及其各基类的转换函数，并由重载决议选取最佳匹配。然后用选取的用户定义转换，转换初始化器表达式为所初始化的对象。
</li><li> 否则，如果 <code>T</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 而原类型是 <span class="t-lc"><a href="../types/nullptr_t.html" title="cpp/types/nullptr t">std::nullptr_t</a></span>，则被初始化对象的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>。
</li><li> 否则，在必要时使用<a href="implicit_cast.html" title="cpp/language/implicit conversion">标准转换</a>，转换 <span class="t-spar">其他对象</span> 的值为 <code>T</code> 的无 cv 限定版本，而所初始化的对象的初值为（可能为转换后的）该值。
</li></ul>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>直接初始比复制初始化更宽松：复制初始化仅考虑非 <a href="explicit.html" title="cpp/language/explicit">explicit</a> 构造函数和非 explicit 的用户定义<a href="cast_operator.html" title="cpp/language/cast operator">转换函数</a>，而直接初始化考虑所有构造函数和所有用户定义转换函数。
</p><p>在使用直接初始化语法 <span class="t-v">(1)</span>（带圆括号）的变量声明和<a href="function.html" title="cpp/language/function">函数声明</a>之间有歧义的情况下，编译器始终选择函数声明。此消歧义规则有时是反直觉的，并且已被称为<a href="https://en.wikipedia.org/wiki/Most_vexing_parse" class="extiw" title="enwiki:Most vexing parse">最烦人的分析</a>。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iterator&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;fstream&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/basic_ifstream.html"><span class="kw1732">std::<span class="me2">ifstream</span></span></a> file<span class="br0">(</span><span class="st0">"data.txt"</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="co1">// 下面是函数声明：</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">(</span><a href="../iterator/istreambuf_iterator.html"><span class="kw669">std::<span class="me2">istreambuf_iterator</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">(</span>file<span class="br0">)</span>,
                    <a href="../iterator/istreambuf_iterator.html"><span class="kw669">std::<span class="me2">istreambuf_iterator</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="co1">// 它声明名为 str 的函数，其返回类型为 std::string，</span>
    <span class="co1">// 第一参数拥有 std::istreambuf_iterator&lt;char&gt; 类型和名称 "file"</span>
    <span class="co1">// 第二参数无名称并拥有类型 std::istreambuf_iterator&lt;char&gt;()，</span>
    <span class="co1">// 它被重写成函数指针类型 std::istreambuf_iterator&lt;char&gt;(*)()</span>
 
    <span class="co1">// C++11 前的修正：用额外括号环绕其中一个实参</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">(</span> <span class="br0">(</span><a href="../iterator/istreambuf_iterator.html"><span class="kw669">std::<span class="me2">istreambuf_iterator</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">(</span>file<span class="br0">)</span> <span class="br0">)</span>,
                      <a href="../iterator/istreambuf_iterator.html"><span class="kw669">std::<span class="me2">istreambuf_iterator</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>  
    <span class="co1">// C++11 后的修正：列表初始化任意实参</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">(</span><a href="../iterator/istreambuf_iterator.html"><span class="kw669">std::<span class="me2">istreambuf_iterator</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">{</span>file<span class="br0">}</span>, <span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>类似地，在以函数式转型表达式 <span class="t-v">(3)</span> 为其最左子表达式的表达式语句，和声明语句间有歧义的情况下，以将它当做声明解决歧义。此消歧义是纯语法的：它不考虑语句中出现的名字除了是否为类型名之外的含义。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> M <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> L <span class="br0">{</span> L<span class="br0">(</span>M<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
M n<span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    M<span class="br0">(</span>m<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 声明，等价于 M m;</span>
    L<span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非良构的声明</span>
    L<span class="br0">(</span>l<span class="br0">)</span><span class="br0">(</span>m<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 仍然是声明</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;memory&gt;</span>
 
<span class="kw1">struct</span> Foo <span class="br0">{</span>
    <span class="kw4">int</span> mem<span class="sy4">;</span>
    <span class="kw1">explicit</span> Foo<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="sy4">:</span> mem<span class="br0">(</span>n<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s1<span class="br0">(</span><span class="st0">"test"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 自 const char* 的构造函数</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s2<span class="br0">(</span><span class="nu0">10</span>, <span class="st0">'a'</span><span class="br0">)</span><span class="sy4">;</span>
 
    <a href="../memory/unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> p<span class="br0">(</span>new <span class="kw4">int</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：允许 explicit 构造函数</span>
<span class="co1">//  std::unique_ptr&lt;int&gt; p = new int(1); // 错误：构造函数为 explicit</span>
 
    Foo f<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// f 被直接初始化：</span>
              <span class="co1">// 构造函数形参 n 从右值 2 复制初始化</span>
              <span class="co1">// f.mem 从形参 n 直接初始化</span>
<span class="co1">//  Foo f2 = 2; // 错误：构造函数为 explicit</span>
 
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s1 <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> s2 <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>p <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> f.<span class="me1">mem</span>  <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">test aaaaaaaaaa 1 2</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="copy_elision.html" title="cpp/language/copy elision"> 复制消除</a>
</li><li> <a href="converting_constructor.html" title="cpp/language/converting constructor"> 转换构造函数</a>
</li><li> <a href="copy_assignment.html" title="cpp/language/copy assignment"> 复制赋值</a>
</li><li> <a href="copy_constructor.html" title="cpp/language/copy constructor"> 复制构造函数</a>
</li><li> <a href="default_constructor.html" title="cpp/language/default constructor"> 默认构造函数</a>
</li><li> <a href="destructor.html" title="cpp/language/destructor"> 析构函数</a>
</li><li> <a href="explicit.html" title="cpp/language/explicit"> explicit</a>
</li><li> <a href="initialization.html" title="cpp/language/initialization"> 初始化</a>
<ul><li> <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization"> 聚合初始化</a>
</li><li> <a href="constant_initialization.html" title="cpp/language/constant initialization"> 常量初始化</a>
</li><li> <a href="default_initialization.html" title="cpp/language/default initialization"> 默认初始化</a>
</li><li> <strong class="selflink"> 直接初始化</strong>
</li><li> <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect"> 初始化器列表</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization"> 列表初始化</a>
</li><li> <a href="reference_initialization.html" title="cpp/language/reference initialization"> 引用初始化</a>
</li><li> <a href="value_initialization.html" title="cpp/language/value initialization"> 值初始化</a>
</li><li> <a href="zero_initialization.html" title="cpp/language/zero initialization"> 零初始化</a>
</li></ul>
</li><li> <a href="move_assignment.html" title="cpp/language/move assignment"> 移动赋值</a>
</li><li> <a href="move_constructor.html" title="cpp/language/move constructor"> 移动构造函数</a>
</li><li> <a href="new.html" title="cpp/language/new"> new</a>
</li></ul>

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